How are empty sets and null sets different?
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From a data point of view NULL is not necessarily = blank.
blank data can be an empty string, a blank text field, a 0, or any other data representing blank data.
NULL is the data not existing. This can be for the same reasons that you have blank data (e.g. data not filled in), but can also represent missing data. Missing data might be an error in the data collection, data deliberately removed, or data not provided for any number of business reasons.
It trips a lot of inexperienced database scripters off guard
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